Crop harvesting machines, like windrowers and combines use various driven elements to effect the cutting and harvesting systems including a cutting system and commonly a reel. The speed of the cutting system is generally variable to accommodate different crop conditions and different harvesting speeds. The reel is used to lift crop that is down & lodged so the cutting mechanism has good access to the plant (not leaving any seeds or desirable plant material behind) and to push the crop into the header for conveying to the windrow discharge. For maximum efficiency (effectively raise the plants, minimize seed shatter, minimize wear on mechanical parts and minimize power required to drive the reel), it is highly desirable that the reel run at the right speed and that the operator has full control of the reel speed for all crop and ground speed conditions.
Reel speed controlled relative to vehicle ground speed has been used on combines for some time to assist the operator to obtain the right speed without excessive input from the operator (which causes fatigue).
Speed sensors are commonly used to provide feedback to a control system and or the machine operator via a digital display. The operator can then monitor and control the operating speed of the item by means of a switch or potentiometer. Using the knife drive speed sensor as an example, the system would normally function as follows:
The operator has a digital readout of the operating speed and through keypad entry on the dash display, the desired speed/rpm of the knife can be set. Once the desired speed has been programmed into the display, the control module continually reads this speed signal and compares it to the desired set point. The control module then outputs a PWM control signal which is used to drive a PWM controlled hydraulic valve. In turn the hydraulic valve varies the flow to the knife drive motor and thus controls the speed of operation. When there is a predetermined amount of deviation from the set point, the control module programming determines how much voltage or current change (+/−) is required in the PWM valve drive signal to bring the actual speed back the desired speed. As long as a signal is being received from the speed sensor, the control module can adjust the output to control the speed. However, if the speed sensor fails, the control system has no way to dynamically determine how to correct the speed when a change is required. The operator could still input a different set point, but the control module would not know how much of a PWM control signal is required to maintain the desired set point.